This invention relates to an impulse wrench of the type having a pneumatic rotation motor with a rotor, power supply means for connecting the motor to a pressure air source, a hydraulic torque impulse generator which is drivingly coupled to the forward end of the motor, and an output shaft for delivering torque impulses to screw joints to be tightened.
A problem concerned with this type of tool is the difficulty to monitor and control the tightening processes in an accurate and reliable way. The reason is that it is difficult to obtain a reliable torque impulse reflecting signal from the impulse delivering tool.
One known way of solving this problem is to use a contact free torque detecting means at the output spindle of the wrench, as described in EP 0 502 748. This known device comprises a specially designed output spindle which at least partly is made of a magnetostrictive material, and a pair of coils surrounding the spindle for detecting torque related distortion of the spindle.
This known torque impulse detecting means requires a modified impulse mechanism with a longer output spindle and an enlarged housing diameter.
As described in two German scientific studies, namely "Diplomarbeit im Fach Steurungs- und Regelungstechnik" from February 1992 and July 1992, the torque impulse detecting problem is solved by a means which does not require any modification of the impulse mechanism itself, but is easily adaptable to impulse wrench designs including impulse magnitude responsive shut-off means. Prior art impulse wrench designs suitable for this previously described technique are disclosed in for instance EP 0 441 758 and U.S. Pat. No. 4,418,764.
According to the abovementioned German studies, a torque impulse reflecting signal is obtained by an inductive displacement detecting device (LVDT=Linear Variable Differential Transformer) coupled to the coaxially extending actuating rod and being located at the rear end of the motor. The impulse wrenches used for these studies are of the type having a mechanical retardation responsive means connected to the impulse generator for obtaining a linear signal generating movement. This means that the signal obtained is an indirect reflection of the torque impulse character, since it is the retardation characteristics of the impulse generator that are actually measured. However, this signal reflects accurately enough the impulse characteristics and is used in a process monitoring and control unit to calculate tightening data of interest, like the shut-off point.
Tightening shut-off is obtained by activation of an external electromagnetic air shut-off valve.
One of the impulse wrenches used in the German studies, "Diplomarbeit" of July 1992, section 2.4.3, is of the type described in EP 0 441 758, and modified with an LVDT unit for signal generation.
A problem concerned with the previously described impulse wrenches having retardation activated means for accomplishing a linear movement is the difficulty to get an accurate operation of the mechanical elements. In particular, there is a problem to obtain an accurate movement transfer by the cam means.
Another problem is the nondistinctive power shut-off obtained by the use of an external air shut-off valve. The pressure air volume enclosed in the supply conduit downstream of the valve tends to maintain the motor rotation after the valve is closed. This may cause undesirable extra impulses and a torque overshoot. An external air shut-off valve also impairs the handling of the wrench.